First, the development history and present situation of pile foundation testing
The development of pile foundation has a long history. Dating back to AD 247, the earliest application of piles began with the stone shore wall of Shanghai Longhua Building and the Hangzhou Bay seawall built in the 10th century. /kloc-In the late 20th century, cement, steel bars and concrete appeared. With the continuous improvement and perfection of mechanical equipment, the shape of pile foundation in high-rise buildings is gradually updated and the styles become diverse. Followed by the in-depth development of pile foundation theory research. Through the renewal and deepening of the theory, it can better guide the pile foundation detection technology in practice.
Pile foundation is one of the commonly used foundation forms of engineering structures and belongs to underground concealed engineering. The construction technology is complex and the process flow is closely related. A little carelessness in construction will easily lead to various complex quality defects such as broken piles, which will affect the integrity and bearing capacity of piles, thus directly affecting the safety of superstructure. Therefore, its quality inspection has become an important means of quality control of pile foundation engineering.
In the development history of pile foundation detection, the update of detection technology has become a wide and hot topic. In order to meet the increasingly complex and exquisite requirements of pile foundation testing, domestic researchers continue to introduce and learn foreign advanced technology, and constantly develop and improve pile foundation technology. With the continuous improvement of infrastructure construction requirements, the size of piles is getting bigger and bigger, and the quality requirements of piles are getting higher and higher, and there are more and more possible problems. Although the development of domestic pile foundation detection technology can not meet all the needs of production, on the whole, the technology and methods of domestic pile foundation detection development are constantly updated and improved.
Second, the method of pile foundation detection
According to the Technical Code for Detection of Building Foundation Piles (JGJ 106-2003), the main methods of pile foundation detection at present include static load test, core drilling method, low strain integrity detection, high strain dynamic detection and acoustic wave transmission method.
1. Static load test method
Static load test method is currently recognized as the most direct and reliable test method to detect the vertical compressive bearing capacity of foundation piles. Before the breakthrough of Wood pile foundation detection technology, the static load test method was considered irreplaceable. Its advantages are direct, simple, reliable and safe. However, in engineering practice, it is found that the problems of benchmark piles are sometimes ignored by inspectors, which are prone to problems such as insufficient driving depth of benchmark piles and displacement during inspection.
Static load test method is also a research topic of great concern to foreign engineering circles. According to the investigation, many scholars at home and abroad have made many attempts and experiments. Especially after 1980s, with the continuous development of economic construction, the static load test method of pile foundation in China has entered a brand-new development period. At present, the static load test method has become an undisputed method in theory and generally recognized in the technology and method of pile foundation detection.
2. Core drilling method
Core drilling method is also called core drilling test method. This method is scientific, intuitive and practical, and has been widely used in the detection of concrete cast-in-place piles. The macroscopic purpose of core drilling method is to detect the length of cast-in-place pile, the concrete strength of pile body and the thickness of sediment at the bottom of pile. This method can well judge and identify the characteristics of rock and soil at the pile tip and accurately judge the integrity category of the pile.
A complete and successful core drilling test can obtain the pile length, concrete strength, sediment thickness at the bottom of the pile and the integrity of the pile, and judge or identify the geotechnical properties of the bearing layer at the pile end. Core-pulling technology has great influence on detection and judgment. In a project, XY- 1 engineering drilling rig is used first, cemented carbide single-pipe drilling tool is used, and the drilling method combining low pressure, slow speed and small pump capacity with dry drilling is adopted. Results The core recovery rate was less than 70%, and the integrity of core samples was extremely poor, and most of them were fragments. Later, SCZ- 1 hydraulic drilling rig was used, and diamond single-acting double-tube drilling tools were used. The core recovery rate reaches 99%, and the core sample is relatively complete cylindrical. Therefore, the "Technical Specification" has made corresponding provisions for drilling rigs and drill bits to avoid misjudgment of core-pulling pile inspection. At present, the technical content of drilling rig equipment has been improved, from single low efficiency to high efficiency multi-functional drilling rig.
3. Low strain integrity test
Low-strain dynamic detection method is also called low-strain reflected wave method (stress wave method), which strikes the top of the pile with a hammer or stick, giving the pile a certain amount of energy, generating a longitudinal stress wave, which propagates downward along the pile body, and the sensor (speed or acceleration type) picks up the pile defects and the reflected signals of different interfaces. By detecting and analyzing the propagation process of stress wave in pile body, the integrity of pile body can be analyzed. According to the reflection wave and transmission wave generated when the interface of the pile body changes suddenly (such as thick sediment at the bottom of the pile, mud inclusion in the pile body, fracture, diameter enlargement or diameter reduction, etc.). ), determine the defect nature of the pile body, estimate the pile length or defect position, and infer the strength of concrete according to the propagation speed of stress wave in the pile body.
In 1980s, low-strain integrity testing entered a period of rapid development, and various low-strain integrity testing made many achievements in basic theory, mechanism, instrument research and development, field testing and signal processing technology, verification research of engineering piles and model piles, and accumulation of practical experience. The low-strain dynamic testing method is simple and quick, but how to obtain a good waveform and how to analyze the integrity of the pile is the key to the testing work. The test process is the key to get a good signal. Please pay attention to: 1. Selection of test points. The number of test points varies according to different pile diameters and different test signals. Generally, the pile diameter is required to be above 120cm, and 3 ~ 4 test points are needed. 2. Selection of hammering point. Regardless of the pile diameter, the hammering point should be 20 ~ 30 cm away from the sensor. 3. Sensor installation. The sensor should be installed according to the position of the selected test point, and the pasting method should be carefully selected. Generally, there are paraffin, butter and plasticine when the pile head is dry and there is no accumulated water. 4. Collect as many signals as possible. A pile of not less than 10 hammer. Observe the consistency of waveforms at different points and under different excitation conditions to ensure that the waveforms are true and without omission.
4. High strain dynamic test
High strain dynamic test, also known as pile test method, is a method to determine the bearing capacity of a single pile by using high dynamic load. In China, the development of power piling has a history of nearly one hundred years. The development of dynamic pile testing technology begins with the formula of dynamic piling. At present, the one-dimensional rod wave theory is still used as the basis of high strain dynamic testing at home and abroad.
The main function of high strain dynamic test is to judge whether the vertical compressive bearing capacity of pile meets the design requirements. When judging horizontal integral cracks and precast pile joint defects, high-strain dynamic testing can reasonably judge the degree of defects on the basis of finding out whether these "defects" affect the vertical compressive bearing capacity, and can be used as a supplementary verification means for low-strain integrity testing. At present, in some areas, it has become a common practice to use high-strain dynamic testing to increase the frequency of spot checks on bearing capacity and integrity.
With the continuous development of domestic infrastructure construction, the amount of pile foundation works is also increasing. At present, various types of concrete pouring piles have been widely used in China. However, due to the huge workload of pile foundation detection, the quality is improved with the development of technology. Compared with the traditional static load test method, high strain dynamic test has great advantages in cost and time cost. Therefore, the high strain method has become a widely used detection method in China because of its simple operation and advanced technology.
5. Acoustic wave projection method
Acoustic wave transmission method, commonly known as buried pipe method, is to bury two or more acoustic pipes in cast-in-place piles to transmit and receive acoustic waves. Wave projection method is based on the use of concrete cast-in-place piles and developed on the basis of structural concrete acoustic wave detection technology. As early as 1970s, acoustic wave projection method has been used to detect the integrity of concrete cast-in-place piles. During the propagation of pile concrete, the continuity of concrete is interrupted due to the existence of defects, and sound waves will be reflected, diffracted, refracted, absorbed and attenuated at the interface between the defect area and concrete.
At present, acoustic wave projection method has become an important means of integrity detection technology for concrete cast-in-place piles with its distinctive technical characteristics. At present, it has been widely used in civil building facilities, water conservancy and electric power, industry and railway construction. Compared with other integrity detection methods, acoustic transmission method can carry out comprehensive and detailed detection, and there are basically no other restrictions. However, due to diffusion, transmission and reflection, the detection results will be affected.
Three. conclusion
The second part discusses the advantages and disadvantages of various pile foundation detection methods in detail. In my opinion, the current pile foundation detection technology cannot rely on a single detection method. When judging the quality of building facilities, building infrastructure is a crucial factor. Therefore, in order to ensure the quality of pile foundation, pile foundation detection technology is more important.
In the current pile foundation detection technology, each single detection method has great limitations. Due to the comprehensive consideration and requirements of detection distance, equipment and data processing, a single detection method can not fully meet the needs of various pile types at present. There will be many shortcomings in the application of pile foundation detection technology in actual detection, which will be continuously improved and updated in actual operation.
In a word, in pile foundation detection, it is necessary to combine various detection methods. Only by using their own characteristics and advantages and flexibly using various methods according to the actual situation can we make a comprehensive and accurate evaluation of pile foundation. In addition, the building environment and the level of construction personnel are also external factors that affect the level of detection technology. In practice, we should try to balance all kinds of factors to achieve the best state.
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